The present invention relates generally to the field of measurement and more specifically the present invention relates to a method and apparatus for gathering full field three dimensional (3D) data using phase shift information generated by moire images.
There are a number of optical methods employed today to extract full-field 3D data from a camera image. Stereo techniques utilizing the image disparity between two cameras have been thoroughly investigated in the academic arena. Although the data can be acquired in one camera frame cycle, establishing correspondence for every pixel in the image has proven to be difficult and computationally expensive. To simplify the processing, structured lighting has been incorporated. A few commercial systems known in the art employ multiple laser light stripes imaged by a stereo camera viewing system to generate full field 3D data. The advantages of such a system include being able to acquire data in one camera frame cycle and elimination of some of the calculation complexity of stereo through the use of laser light stripes.
An alternate approach to extracting the full-field three-dimensional data from a camera image has been through the use of moire contouring. Moire contouring is a full-field, non-contact measurement technique. A moire pattern provides a contour map of the imaged surface analogous to the way a topographic map delineates the contour of the land. The generation of images through moire interferometry is done using the interference of light caused by a set of gratings. Two gratings of known pitch are superimposed upon one another. The first creates a shadow or image of parallel lines of light projected on the object. The second is placed in the imaging arrangement used to view the part and superimposed on the image of the first grating seen on the surface, forming a moire fringe pattern.
The moire fringe pattern comprises a plurality of dark fringes with the orientation and distance between the dark fringes being directly related to the surface profile. The resolution and sensitivity of the moire pattern is adjusted by varying the line spacing of the gratings and/or the angles of illumination and viewing on the part. A single moire image can be analyzed to produce a three-dimensional output with depth resolution typically 1/10 of the fringe contour interval. That is, subfringe interpolation is of limited accuracy using only one image. This is especially true if the image surface has texture which can distort the moire fringe information.
The phase shift measurement of moire images is a very effective approach for gathering a full-field 3D data. Through the phase shifting of moire fringes, many problems associated with extracting information from a single moire image can be eliminated. By obtaining a minimum of three sets of fringes, each with a different phase position of the fringe pattern relative to the part surface, it is possible to uniquely calculate the depth information at every point in the moire image. The phase change causes a modulation of light at each image pixel. The resulting sinusoidal light modulations at each image pixel is used to determine the depth information. The depth calculation is invariant to the reflectivity of the surface as long as the light modulation is within the range of the camera's sensitivity (neither dark or saturated).
The limitations of using this approach are that data is gathered over multiple images as the moire pattern is phase shifted. During the gathering of this data the subject surface must remain stationary during a minimum of three different video exposures of the fringe pattern. In the past, to phase shift the moire pattern one of the gratings or the surface itself had to be shifted for each additional phase, potentially causing disruptions in the subject surface and grating position. These disruptions can cause motion blurring and the loss of surface position information. For many applications it would be advantageous to be able to gather the data with just one camera exposure and still be able to obtain the benefits afforded by the phase shifting technique. Therefore it would be desirable to take all three moire phase images simultaneously, providing a snapshot of the part contour.
Previous methods utilizing only one snapshot to obtain a part contour have been reported using color separation and multiple cameras. This color separation approach employs a three color grating viewed through a black and white submaster by a color video camera. The problem with this approach is that good separation of the colors is needed to obtain clear moire patterns at each of the colors. Color separation techniques also have the disadvantage of being sensitive to part coloration. For a bare metal part, the part coloration would typically not be an extreme limitation, but has been found to limit the measurement resolution due to the need to balance the signals from the three color channels. Three separate video cameras, viewing the part either through a common primary lens, each with a separate submaster grating, or with three independent viewing systems adjacent to each other is also a possible means of obtaining three phases of a moire image pattern at one time. With three video cameras, there is a need to synchronize and calibrate the cameras with respect to each other. The challenges of such video balancing often has suggested the use of cooled, very low noise video cameras, which are available. Perhaps the greater challenge is actually making all three moire patterns look the same with a fixed phase relationship between them. Stability requirements for any high resolution moire measurement for this system have been shown to be in the nanometer region, requiring great precision from the mechanical system supporting the cameras.
Accordingly, a need exists for a simple moire phase measurement system immune from mechanical instability and noise problems with the ability to generate 3D topographical information in a single snapshot.